Virtual device models known in the art (e.g., ITU-R Bt.709) and recommended viewing practices known in the art (e.g., SMPTE RP166) are well suited for viewing as provided by a cathode ray tube (CRT) device, which has traditionally been the most common consumer display device. However, these virtual models and viewing practices cannot satisfy all of the multiplicity of viewing devices and viewing situations available in today's market. Current products have a wide range of features that can vary from, for example, very bright displays with dim surround, to front projection displays with dark surround.
Imagery for home video viewing is color corrected typically using studio monitors which are known to be highly accurate, calibrated CRT monitors. Although an excellent display device, in reality, the CRT has less and less in common with the display devices actually used in homes of consumers. Newer display devices used in the home differ in display brightness, color gamut, contrast ratio, spatial and temporal behavior, etc. This is further complicated given that individual display technologies are diverging among themselves by new advances in backlight technology, power management and the like.
In addition, there is a completely new type of home viewing environment emerging with screens of around 100″ in size and more. These new displays have completely new requirements for the color grading process in a home video framework. In some instances, the requirements may be closer to digital cinema requirements than they are to traditional Direct View CRT based home video.
At this moment, there is no technology that offers a satisfactory solution to the multiple facets of consumer display viewing. Currently, colors are decided based on a studio monitor which is assumed to behave according to the virtual device models (e.g., ITU-R Bt.709, SMPTE 240M and the like). In a Quality Control (QC) step, a few consumer type displays may be connected to detect potential problems, however, no solution covering a broader range of consumer displays and viewing situations is currently available.
The consumer marketplace now offers the buyer a choice of several display technologies. In order to illustrate the differences between technologies, an abbreviated collection of the technologies is reviewed herein. It is to be understood that other technologies exist and may include even greater differences.
Plasma technology offers a deep black and a high peak brightness. Plasma suffers from a dependency of peak brightness with white spot size, a so-called “APL” (average picture level) dependency. In many cases, as a consequence, the picture contrast is modified with the average picture level. Furthermore, it is a display technology which, at least at the current moment, features only a limited number of discrete picture levels. A considerable amount of dithering is applied in order to overcome this weakness. This is why spatial resolution may decrease with very low picture brightness. Spatial resolution from medium to high brightness is usually good. Every brand has its own set of phosphors, which in turn determines the color gamut of the set. Plasma screens offer a good screen size for cost ratio for non-projection and non-rear projection devices.
LCD technology is currently a more expensive solution in terms of size for the price. Current implementations with a CCFL (Cold Cathode Fluorescent Light) backlight offer a high brightness for all average picture levels. On the down side of this technology, there is a limited transparency for the on state and insufficient opaqueness for the off state. Some companies therefore implement a dynamic backlight in their latest products. This is very likely to become a common practice in time. The effect is that the dynamic backlight uses the backlight as a secondary modulator for light output of the display. As an effect, a dark scene becomes even darker and bright scenes become brighter, again depending on the “average picture level”. One notable exception is a methodology which proposes a spatial modulation of the backlight by means of LED's.
Furthermore, LCD displays currently exhibit a significant variation in color gamut. This is caused by a momentary transition from low gamut backlights to a variety of new, high color gamut backlights. At the current time, a majority of LCD displays on the market exhibit non-standard color gamut.
CRT displays have traditionally been different in terms of color temperature and dynamic range. Depending on the manufacturer's preference, the color temperature for white varies from 6500K to more than 10000K. This applies for all other displays as well. The dynamic range is determined by the calibration of the CRT, to show or to not show sub-black (e.g., the level of blackness). Average brightness dependencies are also an issue, the higher the spot size, the darker the picture.
Front projection technology has emerged as an interesting alternative for home theatre. It is potentially possible to show the dynamic range and the color gamut of digital cinema projection, and the viewing condition is basically similar to the one used for digital cinema. However, different display technologies are also used for front projection systems and therefore different results can be expected from different devices.
These display technologies may have very different attributes. In some aspects, these technologies even oppose each other. One exemplary attribute is the average picture level dependency which is basically opposite between Plasma technology and the latest LCD displays. Another exemplary attribute may be the spatial resolution versus brightness on CRT versus Plasma.
The purpose of color correction is to predict what the consumers are going to see and to change the colors in a way to achieve the original artistic intent on the output medium. Knowing that the output medium (i.e., the display used for color correction) used to produce the color correction may have little in common with the output displays in the field, this becomes a difficult, perhaps unsuccessful, or at least an inefficient approach.
Asking the display manufacturers to build displays that better resemble the display currently used for color correction is not an option. This would result in the picture quality suffering because, in general, at least one property of the reference device would be impossible to achieve. In addition, many displays would have to “down grade” to the standard display specification which may have been defined at an earlier time without regard to improved technological advances that occur during the interim time.